The present invention relates to a tumbler dryer with a container for articles to be dried (such as clothes or other fabrics), which is arranged to be traversed by hot air during the drying process and where, at the beginning of the drying process, the temperature of the exhaust air from the container increases relatively rapidly and thereafter increases more slowly or remains constant, and where, towards the end of the drying process, the rate of increase of the temperature starts increasing.
In tumbler dryers it is of importance to be able to terminate the drying process when the moisture content of the articles has reached the desired value. If the drying is interrupted too early, the moisture content is too high with the consequent apparent disadvantages. If the drying is interrupted too late, this involves an unnecessarily high energy consumption, and the too low moisture content of the articles will increase their propensity to wrinkle and make them difficult to iron, and further the risk of static electric charging of the articles will increase. There are therefore considerable advantages to be gained if the drying process can always be terminated at such a time that the dried articles have the desired moisture content. The desired value of the moisture content may vary between, for example, 1-3% (for normal dry laundry) and 8-11% (for iron dry laundry), depending on the subsequent treatment to which the dried laundry is intended to be subjected.
It is previously known to provide tumbler dryers with a simple time program which interrupts the drying after a preset time. Since the rate of drying is very much dependent on several factors, for example the amount of articles to be dried in the container and the material of the articles, it is impossible in practice to predict and set, in a certain case, the drying time which in that particular case gives the desirable moisture content. The dried laundry will therefore have too high or (more often) too low a moisture content, resulting in the above-mentioned disadvantages.
Further, it is known to arrange a temperature sensor which senses the temperature of the exhaust air from the drum-washer. When the tumbler dryer is started, this temperature first increases relatively rapidly and then becomes constant or increases more slowly, while the main portion of the water content of the laundry is driven off. Towards the end of the drying, the temperature starts rising again (or starts rising more rapidly). It is known to arrange members which are influenced by the temperature sensor and which automatically terminate the drying process (the supply of hot air to the drum) when the temperature of the exhaust air during the final stage of the drying has reached a predetermined value, for example 60.degree. C., which may then be suitably selected so that a certain moisture content in the laundry is obtained at full load of the tumbler dryer. However, there is no clear relationship between moisture content and switch-off temperature, and therefore it is not possible in each individual case, by setting the switch-off temperature, to determine what moisture content the laundry will have at the end of the drying. Further, it has proved that when the drum of the machine is only partly filled, the drying will take place more rapidly, and when the exhaust air reaches the switch-off temperature, the moisture content of the laundry is considerably lower than what is the case at full load. At partial load, the drying will therefore be interrupted too late, which results in the disadvantages mentioned in the introduction.
It is also known to sense the temperature of the articles to be dried and the temperature of the air current, and terminate the drying when the difference between these two temperatures drops below a predetermined limit value. In another known method, the drying is terminated when the rate of change of the temperature of the articles or of the exhaust air exceeds a predetermined value. Both these methods involve the drawback that the moisture content of the articles towards the end of the drying becomes greatly dependent on the amount of articles in the container.
To avoid the drawbacks with the above-mentioned methods, it is known to arrange electrodes in the drum and measure the resistance between the electrodes continuously during the drying process. During the rotation of the drum the articles are brought into contact with the electrodes, and the measured resistance becomes dependent on the resistivity of the articles and thus on the moisture content of the articles. As the drying progresses the moisture content is reduced, the resistivity increases and thus the measured resistance value also increases. When this value has reached a predetermined level, the drying is interrupted. However, this method involves several disadvantages. Particularly at low moisture content, it is difficult or impossible to determine, with sufficient accuracy, the moisture content by measuring the resistance in the described manner. Further, the mechanical-electrical design will be complicated (slip rings are required for transmitting the measuring signal). In case of mixed load, for example synthetic and cotton garments, the synthetic garments will dry first. It has proved that in this case the drying is normally not interrupted until the last cotton garment has become dry. The synthetic garments will then become dried to too great an extent, and synthetic material is often very sensitive to this. Another disadvantage of the method of measuring resistance is that the measured resistance, in addition to being influenced by the moisture content of the articles, may also be greatly influenced by lime deposits, etc., on the measuring electrodes and by the conductivity and the degree of purity of the water.
The invention aims to provide a tumbler dryer in which, in a simple and reliable manner and with great accuracy, a preset moisture content of the articles is obtained at the end of the drying process. The resulting moisture content shall be able to be set at different values. It should, as far as possible, be independent of the amount of articles in the drum and of the material of the articles, and it should also be independent of the temperature of the inlet air, which may be influenced by variations in the mains voltage, etc.
What characterises a tumbler dryer according to the invention will be clear from the appended claims.